1. Field of the invention
This invention relates to a chromating solution that can form a chromate film from which hexavalent chromium ions may little dissolve out, and a chromated metal sheet obtained using such a chromating solution.
2. Description of related art
In recent years, as chromating solutions for metal sheets such as steel sheets coated with zinc, aluminum or an alloy of these, copper-coated steel sheets and aluminum sheets, coating types are prevailing in which an aqueous solution of a water-soluble chromium compound such as a chromic acid and a chromate is coated on a metal sheet followed by drying without washing with water to form a chromate film. If chromium ions contained in such a chromating solution are all held by hexavalent chromium ions, the chromate film formed tends to be a film from which hexavalent chromium ions may dissolve out when the metal sheet is treated, and also the film may be moisture-absorptive. Accordingly, as disclosed in, e.g., Japanese Patent Applications Laid-open No. 59-31872 (a method of chromating galvanized steel sheets) and No. 3-219087 (a chromating solution for galvanized steel sheets), usually the hexavalent chromium ions are partly reduced with a reducing agent to insoluble, trivalent chromium ions so that the chromate film can be improved in corrosion resistance and moisture resistance.
As the reducing agent, organic compounds such as monohydric or polyhydric alcohols, hydrocarbons and polysaccharides and inorganic compounds such as hydrogen peroxide and hydrazine are conventionally used. When, however, these reducing agents are used to reduce hexavalent chromium ions to trivalent chromium ions at a reduction percentage of more than 50%, the trivalent chromium ions turn into chromium hydroxide because the pH increases with an increase in reduction percentage, to cause gelation and sedimentation of the chromating solution, making it difficult for the solution to be coated on metal sheets. Hence, the reduction percentage of hexavalent chromium ions has been controlled to be not more than 50%. At such a low reduction percentage, however, the hexavalent chromium ions present in the chromate film may dissolve out to produce yellow stains or to adversely affect handlers, bringing about problems.